Light modulator

ABSTRACT

A light modulator includes a semiconductor substrate having a main surface, a rear surface, and a grounding conductor on the rear surface. A wave guide section having a width is located on the semiconductor substrate. A bonding pad section on the semiconductor substrate is located adjacent to the wave guide section and an insulating layer covers the main surface of the semiconductor substrate. A portion of the insulating layer immediately opposite the bonding pad section includes a multiple layer structure of insulating films. An electrode opposite the bonding pad section is electrically connected to the wave guide section.

This application is a continuation of U.S. application Ser. No.09/245,838, filed Feb. 8, 1999, now U.S. Pat. No. 6,282,009.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a light modulator formodulating a laser beam and, more particularly, to a high speed lightmodulator of a kind used in a high speed optical fiber communicationsystem. The present invention also relates to a method of manufacturingsuch light modulator.

2. Description of the Prior Art

In the high speed optical fiber communication system, a considerableamount of data are transmitted by the use of semiconductor laser beamsand optical fibers. In order to cope with this feature, thesemiconductor laser beams are required to be modulated at a high speed.With the conventional direct modulation system in which the electriccurrent injected to a single-mode semiconductor laser is modulated toprovide the modulated output laser beam, change in wavelength resultingfrom change in density of injected carriers (i.e., wavelength chirping)is so substantial that the conventional direct modulation system cannotbe used in high-speed modulation of 10 Gbps or higher.

In view of the foregoing, as an alternative to the direct modulationsystem, the external modulation system has come to be the cynosure ofthose concerned, in which a light modulator having a low chirping anddisposed externally of a semiconductor laser is utilized to modulate thelaser beam while the current injected to the semiconductor laser isfixed. The combined modulator and laser assembly in which a lightmodulator, a single-mode semiconductor laser and an isolator separatingthe light modulator and the semiconductor laser from each other areintegrated together on a single chip is shown by 60 in FIG. 7. Since nocircuit is required between the modulator and the laser, the combinedmodulator and laser assembly 60 shown therein has a high practicalutility and is extremely important as a key device for optical fibercommunication of a large amount of data.

The light modulator will now be described. As shown in FIG. 8A, thelight modulator 70 includes an InP semiconductor substrate 52 on which asemiconductor mesa layer 56 of a predetermined width containing a lightabsorption layer 51 and a semiconductor bonding pad layer 55 are formed.The laser beam inputted to the light modulator 70 is modulated by thelight absorption layer 51. More specifically, by applying a voltage tothe bonding pad electrode 55 a, an electric field is applied from anelectrode 54, covering the semiconductor mesa layer 56, to the lightabsorbing layer 51, and by shifting the absorption wavelength of thelight absorbing layer 51, the input laser beam is modulated.

As shown in FIG. 8B, a groove 57 is formed between the semiconductormesa layer 56 and the semiconductor bonding pad layer 55 for separatingthe semiconductor layers 55 and 56 from other semiconductor layers. Thesemiconductor mesa layer 56, the semiconductor bonding pad layer 55 andthe groove 57 has their respective surfaces covered by a continuousinsulating film 53. The bonding pad electrode 55 a and the electrode 54are formed by a metallic film continuously covering the insulating film53 while the electrode 54 is held in ohmic contact with thesemiconductor mesa layer 56 through an opening defined in the insulatingfilm 53.

The conventional method of manufacturing the conventional lightmodulator is shown in FIGS. 9A to 9C. Referring first to FIG. 9A, apredetermined crystalline layer is epitaxially grown on the InPsubstrate 52 to form the semiconductor mesa layer 56 of thepredetermined width including the light absorption layer 51, the groove57 and the semiconductor bonding pad layer 55. Then, as shown in FIG.9B, the insulating film 53 of SiO₂ having a film thickness of about 4000Å is formed so as to cover the entire surface of the InP substrate 52.After a window for the ohmic contact has been formed in an upper surfaceof the semiconductor mesa layer 54 including the light absorption layer51, the metallic film is formed at a predetermined location as shown inFIG. 9C to complete the bonding pad electrode 55 a and the electrode 54.

In order for the light modulator to be used for high-speed modulation,it is necessary to reduce the static capacitance (hereinafter referredto as a “parasitic static capacitance”) formed between surfaceelectrodes (the bonding pad electrode 55 a and the electrode 54) and arear surface electrode. The parasitic static capacitance of the lightmodulator is expressed by the sum of the parasitic static capacitance ofthe mesa layer 56 plus the parasitic static capacitance of the bondingpad layer 55. In order to reduce the parasitic static capacitance of thelight modulator, attempts have currently been made to minimize thesurface area of each of the mesa layer 54 and the bonding pad layer 55by forming the groove 57 therebetween.

It has, however, been found that considering the chirping of light thatis propagated by the light absorption layer 51, the width of the mesalayer 56 can only be reduced to a certain limited dimension. Also,considering the bonding surface area of the bonding wire, the size ofthe bonding pad layer 55 is limited to about 50×50 μm. Thus, theapproach to reduce the surface area of the mesa layer 56 and the bondingpad layer 55 in an attempt to reduce the parasitic static capacitance islimited and, therefore, a sufficiently high-speed modulationcharacteristic has been difficult to accomplish.

SUMMARY OF THE INVENTION

The present invention has therefore been developed in view of theforegoing problems and is intended to provide an improved lightmodulator capable of accomplishing a high-speed light modulation inwhich the parasitic static capacitance is reduced and also to provide animproved method of manufacturing such light modulator.

The light modulator of the present invention is such that the parasiticstatic capacitance of the bonding pad section has been reduced tosubstantially eliminate the above discussed problems, and is thereforeeffective to achieve the high-speed modulation. More specifically, thelight modulator of the present invention includes a semiconductorsubstrate having first and second surfaces opposite to each other with agrounding conductor formed on the second surface thereof. A mesa sectionof a predetermined width laminated with a semiconductor layer includinga light absorption layer and a bonding pad forming section adjacent themesa section are formed on the semiconductor substrate. An insulatinglayer continuing from the mesa section to the bonding pad section isformed with an opening defined in a portion of the insulating film abovethe mesa section, and an electrode contacting an upper surface of themesa section through the opening and extending to the bonding padforming section is formed over the insulating layer. Accordance with thepresent invention, the light modulator is featured in that a portion ofthe insulating layer the bonding pad forming section has a thicknessgreater than that of the remaining portion of the insulating layer toreduce the parasitic static capacitance of the bonding pad section.

The portion of the insulating layer immediately above the bonding padforming section comprises a multi layered structure containing at leastinsulating films laminated one above other. The remaining portion of theinsulating layer comprises a single or multi layered structurecontaining a insulating films, in which a number of the insulating filmis less than that of the bonding pad forming section.

The insulating films are two insulating films, one of the two insulatingfilms is made of SiO₂ and the other is made of SiN.

The upper-layer insulating film of remaining portion of the insulatinglayer is same as the 2nd upper-layer insulating film of the bonding padforming section.

The first method of manufacturing the light modulator according to thepresent invention is such that the parasitic static capacitance of thebonding pad section has been reduced to substantially eliminate theabove discussed problems. More specifically, this first method isutilized to manufacture the light modulator which includes asemiconductor substrate having first and second surfaces opposite toeach other with a grounding conductor formed on the second surfacethereof, which substrate is formed with a mesa section of apredetermined width, laminated with a semiconductor layer including alight absorption layer, and a bonding pad forming section adjacent themesa section, an insulating layer continuing from the mesa section tothe bonding pad section and formed with an opening defined in a portionof the insulating layer above the mesa section, and a one-pieceelectrode formed over the insulating film and contacting an uppersurface of the mesa section through the opening, the one-piece electrodeforming a bonding pad electrode. This first method is featured in thatit comprises forming a primary insulating film continuing from the mesasection to the bonding pad forming section, forming a mask so as tocover a portion of the primary insulating film formed above the bondingpad forming section, etching the primary insulating film to removeanother portion of the primary insulating film other than that portionof the primary insulating film above the bonding pad forming section,removing the mask forming a secondary insulating film continuing fromthat portion of the primary insulating film above the bonding padforming section and the mesa section and completing the insulatingwhereby that portion of the insulating layer above the bonding padforming section has a thickness greater than that of the remainingportion of the insulating layer to reduce the parasitic staticcapacitance of the bonding pad section.

The second method of manufacturing the light modulator according to thepresent invention is such that the parasitic static capacitance of thebonding pad section has been reduced to substantially eliminate theabove discussed problems. More specifically, this second method isutilized to manufacture the light modulator which includes asemiconductor substrate having first and second surfaces opposite toeach other with a grounding conductor formed on the second surfacethereof, which substrate is formed with a mesa section of apredetermined width, laminated with a semiconductor layer including alight absorption layer, and a bonding pad forming section adjacent themesa section, an insulating layer continuing from the mesa section tothe bonding pad section and formed with an opening defined in a portionof the insulating layer above the mesa section, and a one-pieceelectrode formed over the insulating film and contacting an uppersurface of the mesa section through the opening, the one-piece electrodeforming a bonding pad electrode. This second method is featured in thatit comprises forming a primary insulating film continuing from the mesasection to the bonding pad forming section, forming a mask so as tocover a portion of the primary insulating film other than a portion ofthe primary insulating film that is formed above the bonding pad formingsection, forming a secondary insulating film over the mask and thatportion of the primary insulating film above the bonding pad formingsection, removing the mask to allow that portion of the secondaryinsulating film above the bonding pad section to continue to thatportion of the primary insulating film above the mesa section to therebycomplete the insulating layer so that that portion of the insulatinglayer above the bonding pad forming section has a thickness greater thanthat of the remaining portion of the insulating layer to reduce theparasitic static capacitance of the bonding pad section.

The third method of manufacturing the light modulator according to thepresent invention is such that the parasitic static capacitance of thebonding pad section has been reduced to substantially eliminate theabove discussed problems. More specifically, this third method isutilized to manufacture the light modulator which includes asemiconductor substrate having first and second surfaces opposite toeach other with a grounding conductor formed on the second surfacethereof, which substrate is formed with a mesa section of apredetermined width, laminated with a semiconductor layer including alight absorption layer, and a bonding pad forming section adjacent themesa section, an insulating layer continuing from the mesa section tothe bonding pad section and formed with an opening defined in a portionof the insulating layer above the mesa section, and a one-pieceelectrode formed over the insulating layer and contacting an uppersurface of the mesa section through the opening, the one-piece electrodeforming a bonding pad electrode. This third method is featured in thatit comprises a primary insulating film forming step of forming a primaryinsulating film continuing from the mesa section to the bonding padforming section, forming a mask so as to cover a portion of the primaryinsulating film above the bonding pad forming section, etching anotherportion of the primary insulating film other than that portion of theprimary insulating film above the bonding pad forming section to apredetermined thickness, removing the mask so as to leave the insulatingfilm having a thick film portion above the bonding pad forming sectionand a thin film portion above the mesa section, the thick and thin filmportion being continued together, and completing the insulating layerwhereby that portion of the insulating layer above the bonding padforming section has a thickness greater than the remaining portion ofthe insulating layer to thereby reduce the parasitic static capacitanceof the bonding pad section.

The primary insulating film forming step of the third method of thepresent invention discussed above may include forming an under-layerinsulating film continuing from the mesa section to the bonding padforming section, forming over the under-layer insulating film anintermediate-layer insulating film of a material different from that ofthe under-layer insulating film, and forming over the intermediate-layerinsulating film an upper-layer insulating film of the same material asthat of the under-layer insulating film. In such case, the etching stepmay be carried out for selectively etching only a portion theupper-layer other than that formed above the bonding pad section, andcompleting the insulating layer.

Preferably, in any of the first to third method, the insulating film ismade of SiO₂ or SiN.

In the practice of the third method, one of the primary insulating filmand the secondary insulating film is preferably made of SiO₂ while theother thereof is preferably made of SiN.

Also, in the practice of any one of the first to third methods of thepresent invention, the insulating film is preferably formed by the useof a CVD technique, a sputtering technique or a vacuum evaporationtechnique.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a fragmentary perspective view of a light modulator accordingto a first preferred embodiment of the present invention;

FIG. 1B is a cross-sectional view taken along the line IB—IB in FIG. 1A;

FIGS. 2A to 2D are respective views similar to FIG. 1B, showing a methodof manufacture of the light modulator according to the first embodimentof the present invention;

FIGS. 3A to 3D are respective views similar to FIG. 1B, showing themodified method of manufacture of the light modulator according to thefirst embodiment of the present invention;

FIGS. 4A to 4D are respective views similar to FIG. 1B, showing thefurther modified method of manufacture of the light modulator accordingto the first embodiment of the present invention;

FIG. 5A is a fragmentary perspective view of a light modulator accordingto a second preferred embodiment of the present invention;

FIG. 5B is a cross-sectional view taken along the line VB—VB in FIG. 5A;

FIGS. 6A to 6D are respective views similar to FIG. 5B, showing themethod of manufacture of the light modulator according to the secondembodiment of the present invention;

FIG. 7 is a perspective view of the prior art combined modulator andlaser assembly;

FIG. 8A is a perspective view of the prior art modulator;

FIG. 8B is a cross-sectional view taken along the line VIIIB—VIIIB inFIG. 8A; and

FIGS. 9A to 9C are view similar to FIG. 8B, showing the prior art methodof manufacturing the prior art light modulator.

DESCRIPTION OF THE EMBODIMENTS First Embodiment

A light modulator according to a first embodiment of the presentinvention is shown by 50 in FIGS. 1A to 4D. Referring particularly toFIGS. 1A and 1B, the light modulator 50 includes a mesa section 9 and abonding pad forming section 6 both protruding outwardly from an InPsemiconductor substrate 2. More specifically, the mesa section 9 isdelimited by and between a pair of parallel grooves 3 formed at apredetermined position in the semiconductor substrate 2. The bonding padforming section 6 is positioned on one side of one of the grooves 3opposite to the mesa section 9 and is delimited by such one of thegrooves 3 and a generally U-shaped groove 3 a defined in thesemiconductor substrate 2. The mesa section 9 includes an electrode 5formed thereon through a SiO₂ insulating layer 4 and has a lightabsorption layer 1 embedded therein, which layer 1 is operable toreceive and transmit a laser beam therethrough. On the other hand, thebonding pad forming section 6 includes a bonding pad electrode 6 aformed thereon through a SiO₂ insulating thick-layer 40 interveningbetween it and the bonding pad electrode 6 a. The bonding pad electrode6 a and the electrode 5 form and are served respectively by differentparts of a metallic layer. It is to be noted that the light modulator 50has a rear surface formed with a rear surface electrode 10.

In the illustrated light modulator 50, the insulating layer 4 and theinsulating thick-layer 40 are integral parts of a single insulatinglayer that are formed above the mesa section 9 and above the bonding padforming section 6, respectively. However, as best shown in FIG. 1B, aportion of the insulating layer overlaying the bonding pad formingsection 6, that is, the insulating thick-layer 40 has a thicknessgreater than the remaining portion of the insulating layer, for example,the insulating layer 4 overlaying the mesa section 9. In addition asdiscussed hereinabove, the insulating layer 4 and the insulatingthick-layer 40 are covered by the metallic layer and a portion of themetallic layer immediately above the bonding pad forming section 6serves as the bonding pad electrode 6 a while another portion of themetallic layer immediately above the mesa section 9 serves the electrode5. The electrode 5 is held in contact with an upper surface of the mesasection 9 through an opening defined in the insulating layer 4.

Since the insulating thick-layer 40 is formed only immediately below thebonding pad electrode 6 a, the parasitic static capacitance between thebonding pad electrode 6 a and the rear surface electrode 10 can bereduced. In detail, by increasing the thickness of thick-layer 40 aroundthe bonding pad forming section 6 from 4000 Å to 8000 Å, the parasiticcapacitance around the bonding pad forming section 6 is half of theparasitic capacitance around the mesa section 9, because the capacitanceis inversely proportional to the thickness of the insulating layer.Also, since the insulating thick-layer 40 is formed on a relativelynarrow portion of the bonding pad forming section 6, there is nopossibility that the substrate 2 may warp. Furthermore, since thatportion of the insulating layer overlaying the mesa section 9 has arelatively small thickness, as is the case with that in the conventionallight modulator, there is no difficulty forming the opening in theinsulating layer.

The method of manufacturing the light modulator 50 of the structurediscussed above will now be described with particular reference to FIGS.2A to 2D. At the outset, as shown in FIG. 2A, after a predeterminedcrystalline layer has been epitaxially grown on the InP semiconductorsubstrate 2, a pair of grooves 3 are formed at a predetermined locationin the semiconductor substrate 2 so that the mesa section 9 of apredetermined width having the light absorption layer 1 can beeventually formed between these grooves 3. The generally U-shaped groove3 a continued to one of the grooves 3 is also formed to define thebonding pad forming section 6. Then, as shown in FIG. 2B, a SiO₂insulating film 41 having a thickness of about 4000 Å is formed on thesemiconductor substrate 2 so as to overlay the mesa section 9 and thebonding pad forming section 6 continuously. Thereafter, as shown in FIG.2C, only a portion of the insulating film 41 overlaying the bonding padforming section 6 is covered by a photo-resist 7 that is utilized as amask, followed by removal of the remaining portion of the insulatingfilm 41 other than that portion of the insulating film 41 overlaying thebonding pad forming section 6. After the subsequent removal of thephoto-resist 7, as shown in FIG. 2D, another SiO₂ insulating film 42having a thickness of about 4000 Å is again formed on the semiconductorsubstrate 2 so as to overlay the mesa section 9 and the bonding padforming section 6 continuously. In this way, the bonding pad formingsection 6 is covered by the insulating thick-layer 40 of a thickness ofabout 8000 Å, i.e., that portion of the insulating film 41 overlapped bythe insulating film 42, while a portion other than the bonding padforming section, for example, the mesa section 9 is covered only by theinsulating film 42 of about 4000 Å.

Thus, the bonding pad electrode 6 a and the electrode 2 are formedimmediately above the bonding pad forming section 6 and the mesa section2, respectively. More specifically, while the opening is formed in theportion of the insulating layer overlaying an upper surface of the mesasection and the metallic layer connected in part with the upper surfaceof the mesa section through the opening and in part overlaying thebonding pad forming section. The rear surface electrode 10 is formed inany known manner as shown in FIG. 1A, thereby completing the lightmodulator 50 of the present invention.

Since as discussed above the insulating thick-layer 40 is formed onlyimmediately below the bonding pad electrode 6 a, the parasitic staticcapacitance between the bonding pad electrode 6 a and the rear surfaceelectrode 10 can be reduced. Also, since the insulating thick-layer 40is formed on that relatively narrow portion of the bonding pad formingsection 6, there is no possibility that the substrate 2 may warp.Furthermore, since that portion of the insulating layer overlaying themesa section 9 has a relatively small thickness as is the case with thatin the conventional light modulator, there is no difficulty forming theopening in the insulating layer.

The light modulator 50 of the present invention can also be manufacturedby an alternative method which will now be described with reference toFIGS. 3A to 3D. Referring to FIG. 3A, after a predetermined crystallinelayer has been epitaxially grown on the InP semiconductor substrate 2, apair of grooves 3 are formed at a predetermined location in thesemiconductor substrate 2 so that the mesa section 9 of a predeterminedwidth having the light absorption layer 1 can be eventually formedbetween these grooves 3. The generally U-shaped groove 3 a continued toone of the grooves 3 is also formed to define the bonding pad formingsection 6. Then, as shown in FIG. 3B, a SiO₂ insulating film 40 a havinga thickness of about 8000 Å is formed on the semiconductor substrate 2so as to overlay the mesa section 9 and the bonding pad forming section6 continuously. Thereafter, as shown in FIG. 3C, only a portion of theinsulating film 41 overlaying the bonding pad forming section 6 iscovered by a photo-resist 7 that is utilized as a mask, followed byetching of the remaining portion of the insulating film 40 a other thanthat portion of the insulating film 40 a overlaying the bonding padforming section 6 to render that remaining portion of the insulatingfilm 40 a to have a film thickness of about 4000 Å. After the subsequentremoval of the photo-resist 7, as shown in FIG. 3D, the insulating layercontinuously covering the semiconductor surface including the bondingpad forming section 6 and the mesa section 9 can be obtained. A portionof the insulating layer overlying the bonding pad forming section 6 andits vicinity has a film thickness of about 8000 Å (the insulatingthick-film 40) while the other portion of the insulating film, forexample, a portion of the insulating film overlaying the mesa sectionhas a thickness of about 4000 Å (the insulating film 4).

Thus, the bonding pad electrode 6 a and the electrode 2 are formedimmediately above the bonding pad forming section 6 and the mesa section2, respectively. More specifically, while the opening is formed in theportion of the insulating layer overlaying an upper surface of the mesasection and the metallic layer connected in part with the upper surfaceof the mesa section through the opening and in part overlaying thebonding pad forming section. The rear surface electrode 10 is formed inany known manner as shown in FIG. 1A, thereby completing the lightmodulator 50 of the present invention.

Since as discussed above the insulating thick-layer 40 is formed onlyimmediately below the bonding pad electrode 6 a, the parasitic staticcapacitance between the bonding pad electrode 6 a and the rear surfaceelectrode 10 can be reduced. Also, since the insulating thick-film 40 isformed on that relatively narrow portion of the bonding pad formingsection 6, there is no possibility that the substrate 2 may warp.Furthermore, since that portion of the insulating layer overlaying themesa section 9 has a relatively small thickness as is the case with thatin the conventional light modulator, there is no difficulty forming theopening in the insulating layer.

A further modified method of manufacturing the light modulator 50 of thepresent will now be described with reference to FIGS. 4A to 4D. At theoutset, as shown in FIG. 4A, after a predetermined crystalline layer hasbeen epitaxially grown on the InP semiconductor substrate 2, a pair ofgrooves 3 are formed at a predetermined location in the semiconductorsubstrate 2 so that the mesa section 9 of a predetermined width havingthe light absorption layer 1 can be eventually formed between thesegrooves 3. The generally U-shaped groove 3 a continued to one of thegrooves 3 is also formed to define the bonding pad forming section 6.Then, as shown in FIG. 4B, a SiO₂ insulating film 41 having a thicknessof about 4000 Å is formed on the semiconductor substrate 2 so as tooverlay the mesa section 9 and the bonding pad forming section 6continuously. Thereafter, as shown in FIG. 4C, only a portion of theinsulating film 41 other than that overlaying the bonding pad formingsection 6, for example, only a portion of the insulating film 41overlaying the mesa section 9 is covered by a photo-resist 7 that isutilized as a mask, followed by deposition of a SiO₂ insulating film 42of about 4000 Å in thickness so as to continuously cover the mesasection 9 and the bonding pad forming section 6. Accordingly, theinsulating film 42 is in part formed over the mask 7 and the bonding padforming section 6 is covered by not only the insulating film 41, butalso the insulating film 42 overlaying the insulating film 41.Subsequent removal of the photo-resist 7 is accompanied by removal ofthat portion of the insulating film 42 overlaying the photo-resist 7,allowing that portion of the insulating film 41 overlaying the mesasection 9 to be exposed to the outside. Consequently, as shown in FIG.4D, the insulating layer continuously overlaying the mesa section 9 andthe bonding pad forming section 6 can be obtained. Since that portion ofthe insulating film 41 overlaying the bonding pad forming section 6 islaminated with a corresponding portion of the insulating film 42, thatportion of the insulating layer above the bonding pad forming section 6has a thickness of about 8000 Å (the insulating thick-layer 40), but theremaining portion of the insulating layer other than that over thebonding pad forming section 6, for example, that overlaying the mesasection has a film thickness of about 4000 Å (the insulating film 41).

Thus, the bonding pad electrode 6 a and the electrode 2 are formedimmediately above the bonding pad forming section 6 and the mesa section2, respectively. More specifically, while the opening is formed in theportion of the insulating layer overlaying an upper surface of the mesasection and the metallic layer connected in part with the upper surfaceof the mesa section through the opening and in part overlaying thebonding pad forming section. The rear surface electrode 10 is formed inany known manner as shown in FIG. 1A, thereby completing the lightmodulator 50 of the present invention.

Since as discussed above the insulating thick-layer 40 is formed onlyimmediately below the bonding pad electrode 6 a, the parasitic staticcapacitance between the bonding pad electrode 6 a and the rear surfaceelectrode 10 can be reduced. Also, since the insulating thick-layer 40is formed on that relatively narrow portion of the bonding pad formingsection 6, there is no possibility that the substrate 2 may warp.Furthermore, since that portion of the insulating film 42 overlaying themesa section 9 has a relatively small thickness as is the case with thatin the conventional light modulator, there is no difficulty forming theopening in the insulating layer.

It is to be noted in any one of the foregoing methods, the insulatinglayer 40 has been described as made of SiO₂, the present invention canbe equally applied where SiN or any other insulating layer is employedtherefor.

Also, formation of the insulating layer may be carried out any knownmethod such as, for example, by the use of the CVD, sputtering or vacuumevaporation technique.

Second Embodiment

The light modulator 50 a according to a second preferred embodiment ofthe present invention is shown in FIGS. 5A and 5B. In this embodiment,the light modulator 50 a includes, as shown in FIG. 5A, a mesa section 9and a bonding pad forming section 6 both protruding outwardly from anInP semiconductor substrate 2. More specifically, the mesa section 9 isdelimited by and between a pair of parallel grooves 3 formed at apredetermined position in the semiconductor substrate 2. The bonding padforming section 6 is positioned on one side of one of the grooves 3opposite to the mesa section 9 and is delimited by such one of thegrooves 3 and a generally U-shaped groove 3 a defined in thesemiconductor substrate 2. The mesa section 9 includes an electrode 5formed thereon through a SiO₂ insulating film 44 and has a lightabsorption layer 1 embedded therein, which layer 1 is operable toreceive and transmit a laser beam therethrough. On the other hand, thebonding pad forming section 6 includes a bonding pad electrode 6 aformed thereon through a SiO₂ insulating film 45 intervening between itand the bonding pad electrode 6 a. The bonding pad electrode 6 a and theelectrode 5 form and are served respectively by different parts of ametallic layer. It is to be noted that the light modulator 50 has a rearsurface formed with a rear surface electrode 10.

In the illustrated light modulator 50 a, as shown in FIG. 5B, a doublelayered structure including a SiO₂ insulating film 43 and a SiNinsulating film 44 is formed on the semiconductor substrate 2 so as tocontinuously cover the mesa section 9, the bonding pad forming section 6and the generally U-shaped groove 3 a. The SiO₂ insulating film 45referred to above is formed over a portion of the SiN insulating film 44overlaying the bonding pad forming section 6. In other words, thebonding pad forming section 6 is covered by an insulating thick-film ofa three layered structure including respective portions of theinsulating films 43, 44 and 45. A metallic layer is formed over theinsulating film 4 and the insulating thick-film so that a portion of themetallic layer overlaying the bonding pad forming section and anotherportion of the metallic layer overlaying the mesa section 9 form thebonding electrode 6 a and the electrode 5, respectively. The electrode 5is held in contact with the mesa section 4 through an opening formedabove an upper surface of the mesa section 4.

Since the insulating thick-film is formed only immediately below thebonding pad electrode 6 a, the parasitic static capacitance between thebonding pad electrode 6 a and the rear surface electrode 10 can bereduced. In detail, by increasing the thickness of thick-layer 40 aroundthe bonding pad forming section 6 from 4000 Å to 8000 Å, the parasiticstatic capacitance around the bonding pad forming section 6 is half ofthe parasitic static capacitance around the mesa section 9.Becase thecapacitance is inverse proportion to the thickness of the insulatinglayer. Also, since the insulating thick-film 40 is formed on arelatively narrow portion of the bonding pad forming section 6, there isno possibility that the substrate 2 may warp. Furthermore, since thatportion of the insulating layer overlaying the mesa section 9 has arelatively small thickness as is the case with that in the conventionallight modulator, there is no difficulty forming the opening in theinsulating layer.

The light modulator 50 a according to the second embodiment of thepresent invention can be manufactured by the following method which willbe described with particular reference to FIGS. 6A to 6D. At the outset,as shown in FIG. 6A, after a predetermined crystalline layer has beenepitaxially grown on the InP semiconductor substrate 2, a pair ofgrooves 3 are formed at a predetermined location in the semiconductorsubstrate 2 so that the mesa section 9 of a predetermined width havingthe light absorption layer 1 can be eventually formed between thesegrooves 3. The generally U-shaped groove 3 a continued to one of thegrooves 3 is also formed to define the bonding pad forming section 6.Then, as shown in FIG. 6B, a SiO₂ insulating film 43 of 2000 Å inthickness, a SiN insulating film 44 of 2000 Å in thickness and a SiO₂insulating film 45 of 4000 Å in thickness are successively formed on thesemiconductor substrate 2 so as to cover the mesa section 9 and thebonding pad forming section 6 continuously. Thereafter, as shown in FIG.6C, only the bonding pad forming section 6 is covered by a photo-resist7 that is utilized as a mask, followed by removal by etching of aportion of the outermost insulating film 45 overlaying the substrate 2,for example, the mesa section 9 and the grooves 3, other than thebonding pad forming section.

Since the insulating film 45 so removed is made of SiO₂ and theinsulating film 44 beneath the insulating film 45 is made of SiN, theuse of an etching solution capable of selectively etching SiO₂ onlywhile the insulating film 44 in the form of the SiN film serves as anetching stopper layer is effective to controllably remove only theoutermost SiO₂ film. For example, the etching soluitin is a hydrofluoricacid, because the etching rate for SiO₂ is as 3-5 times as for SiN.After the subsequent removal of the photo-resist 7, as shown in FIG. 6D,the insulating thick-film, about 8000 Å in thickness, of the threelayered structure including the insulating films 43, 44 and 45 areformed only over the bonding pad forming section 6, while the otherarea, for example, the mesa section 9 and the grooves 3 are covered bythe insulating film, about 4000 Å in thickness, of the double layeredstructure including the insulating layers 43 and 44.

Thus, the bonding pad electrode 6 a and the electrode 2 are formedimmediately above the bonding pad forming section 6 and the mesa section2, respectively. More specifically, while the opening is formed in theportion of the insulating layer overlaying an upper surface of the mesasection and the metallic layer connected in part with the upper surfaceof the mesa section through the opening and in part overlaying thebonding pad forming section. The rear surface electrode 10 is formed inany known manner as shown in FIG. 5A, thereby completing the lightmodulator 50 of the present invention.

It is to be noted in any one of the foregoing methods, the insulatingfilms 43 and 45 has been described as made of SiO₂ and the insulatingfilm 44 has been described as made of SiN, the present invention may notbe always limited thereto and the insulating films 43 and 45 may be madeof SiN and the insulating film 44 maybe made of SiO₂. In this case, Forexample, the etching method is a plasma etching by CF₄ gas, because theetching rate for SiN is over 5 times than for SiO₂.

In the practice of the method of manufacturing the light modulator 50 a,the insulating films or layers are preferably made by the use of anyknown method such as, for example, by the use of the CVD, sputtering orvacuum evaporation technique.

Since as discussed above the insulating thick-film 45 is formed onlyimmediately below the bonding pad electrode 6 a, the parasitic staticcapacitance between the bonding pad electrode 6 a and the rear surfaceelectrode 10 can be reduced. Also, since the insulating thick-layer 40is formed on that relatively narrow portion of the bonding pad formingsection 6, there is no possibility that the substrate 2 may warp.Furthermore, since that portion of the insulating layer overlaying themesa section 9 has a relatively small thickness as is the case with thatin the conventional light modulator, there is no difficulty forming theopening in the insulating layer.

Thus, in the light modulator of the present invention, that portion ofthe insulating layer immediately below the bonding pad electrode has athickness greater than the remaining portion thereof to reduce theparasitic static capacitance of the bonding pad electrode. Accordingly,the light modulator so manufactured can be used for a high-speedmodulation.

The parasitic static capacitance of the bonding pad electrode caneffectively be reduced as a result that that portion of the insulatinglayer immediately below the bonding pad electrode is made of SiO₂ orSiN. Accordingly, the light modulator so manufactured can be used for ahigh-speed modulation.

Since the light modulator of the present invention is such that thelaminated structure of two or more insulating films is formedimmediately below the bonding pad electrode, the selective etchingmethod for forming the insulating layer immediately below the bondingpad electrode is effective to controllably form the insulating layer toa desired thickness.

According to the method of manufacturing the light modulator of thepresent invention, after the primary insulating film continuouslycovering the mesa section and the bonding pad forming section has beenformed, a portion of the primary insulating film other than thatcovering the bonding pad forming section is removed, followed byformation of a secondary insulating film continuously covering the mesasection and the bonding pad forming section. In this way, the insulatinglayer having a great thickness can be formed immediately below thebonding pad electrode.

Also, according to the method of manufacturing the light modulator ofthe present invention, after the primary insulating film continuouslycovering the mesa section and the bonding pad forming section has beenformed, a mask is formed on a portion other than the bonding pad formingsection, followed by formation of a secondary insulating film before theremoval of the mask. In this way, the insulating layer is formed in twostages only over the bonding pad section. In this way, the insulatinglayer having a great thickness can be formed immediately below thebonding pad electrode.

Furthermore, according to the method of manufacturing the lightmodulator of the present invention, after the primary insulating filmcontinuously covering the mesa section and the bonding pad formingsection has been formed, a mask is formed on the bonding pad formingsection, followed by etching of a portion of the insulating film otherthan the bonding pad forming section to a predetermined film thickness.In this way, the insulating layer is formed in two stages only over thebonding pad section. In this way, the insulating layer having a greatthickness can be formed immediately below the bonding pad electrode atone step, not two step. Yet, according to the method of manufacturingthe light modulator of the present invention, the selective etchingtechnique is used to etch the primary insulating film covering the mesasection and the bonding pad forming section continuously. By thisreason, the film thickness of the insulating film can be accuratelycontrolled.

In the practice of the method of manufacturing the light modulator ofthe present invention, the insulating film or films are made of SiO₂ orSiN and, accordingly the parasitic static capacitance of the lightmodulator can effectively be reduced.

Also, in the practice of the method of manufacturing the light modulatorof the present invention, the insulating film or films are made of SiO₂and SiN and, accordingly the parasitic static capacitance of the lightmodulator can effectively be reduced.

The use of the CVD, sputtering or vacuum evaporation technique to formthe insulating film or films is effective to facilitate formation of theinsulating film or films.

Although the present invention has been described in connection with thepreferred embodiments thereof with reference to the accompanyingdrawings, it is to be noted that various changes and modifications areapparent to those skilled in the art. Such changes and modifications areto be understood as included within the scope of the present inventionas defined by the appended claims, unless they depart therefrom.

What is claimed is:
 1. A light modulator comprising; a semiconductorsubstrate having a main surface, a rear surface, and a groundingconductor on the rear surface; a wave guide section having a width andlocated on said semiconductor substrate; a bonding pad section on saidsemiconductor substrate at a location adjacent said wave guide section;an insulating layer covering the main surface of said semiconductorsubstrate, a portion of said insulating layer immediately opposite saidbonding pad section comprising a multiple layer structure includinginsulating films; and an electrode opposite said bonding pad section andelectrically connected to said wave guide section.
 2. The lightmodulator according to claim 1, wherein said insulating films include aninsulating film of silicon oxide and an insulating film of siliconnitride.
 3. The light modulator according to claim 1, wherein one ofsaid insulating films is selected from the group consisting of aninsulating film of silicon oxide and an insulating film of siliconnitride.
 4. The light modulator according to claim 1, wherein said waveguide section includes a light absorption layer and a stripe portion onsaid light absorption layer, said stripe portion having the width.
 5. Alight modulator comprising: a semiconductor substrate having a mainsurface, a rear surface, and a grounding conductor on the rear surface;a wave guide section having a width and located on said semiconductorsubstrate; a bonding pad section on said semiconductor substrate at alocation adjacent said wave guide section; an insulating layer coveringthe main surface of said semiconductor substrate, said insulating layerhaving first and second portions, said first portion of said insulatinglayer being immediately opposite said bonding pad section, andcomprising a multiple layer structure including a plurality ofinsulating films, and having a first thickness, and said second portionof said insulating layer having a second thickness smaller than thefirst thickness; and an electrode opposite said bonding pad section andelectrically connected to said wave guide section.
 6. The lightmodulator according to claim 5, wherein one of said insulating films isselected from the group consisting of an insulating film of siliconoxide and an insulating film of silicon nitride.
 7. The light modulatoraccording to claim 5, wherein said insulating films include aninsulating film of silicon oxide and an insulating film of siliconnitride.
 8. The light modulator according to claim 5, wherein said waveguide section includes a light absorption layer and a stripe portion onsaid light absorption layer, said stripe portion having the width.
 9. Alight modulator comprising: a semiconductor substrate having a mainsurface, a rear surface, and a grounding conductor on the rear surface;a wave guide section having a width and located on said semiconductorsubstrate, a bonding pad section on said semiconductor substrate at alocation adjacent said wave guide section; an insulating layer coveringthe main surface of said semiconductor substrate, said insulating layerhaving first and second portions, said first portion of said insulatinglayer being immediately opposite said bonding pad section and having afirst thickness, said second portion of said insulating layer having asecond thickness, smaller than the first thickness, said first andsecond portions consisting of the same material; and an electrodeopposite said bonding section and electrically connected to said waveguide section.
 10. The light modulator according to claim 9, whereinsaid first and second portions of said insulating layer are selectedfrom the group consisting of silicon oxide and silicon nitride.
 11. Thelight modulator according to claim 9, wherein said wave guide sectionincludes a light absorption layer and a stripe portion on said lightabsorption layer, said stripe portion having the width.